The present invention relates to a laser modulating optical system, which is used in a laser photo plotter or the like, including a multiple line laser source and an acoustooptic modulator (AOM).
The laser photo plotter is provided with a light source that emits a laser beam, a polygon mirror that deflects the modulated laser beam and an f.theta. lens that focuses the deflected laser beam to form a beam spot on an object surface to be exposed, for example. The beam spot scans as the polygon mirror rotates, which forms scanning lines on the object surface.
An optical device such as the laser photo plotter that requires relatively large light quantity employs a gas laser as the light source. In general, a gas laser emits at a plurality of peak wavelengths. For example, an argon laser emits at a plurality of peak wavelengths in the ultraviolet region and the visible region.
On the other hand, the gas laser cannot directly switch (ON/OFF) an emission of the laser beam, which requires a modulator located in the optical path between the light source and the polygon mirror. The AOM, which changes a direction of an emergent laser beam by diffraction caused by input ultrasonic wave, is frequently used as the modulator of the laser photo plotter. In the AOM, when the ultrasonic wave is applied to a medium, a diffraction grating is formed by compression wave caused in the medium. The diffracted light is used as the modulated drawing beam and the non-diffracted light is cut off.
However, since the diffraction angle of the emergent laser beam modulated by the AOM varies in proportion to the wavelength of the incident light, the emergent angles vary depending on the wavelength, which deviates the optical paths of the respective wavelengths. As a result, if the AOM is not conjugate to the object surface, the laser beam that should form a single beam spot forms a plurality of beam spots at the different points on the object surface. If the AOM is conjugate to the object surface, the position variation of the beam spots can be prevented. However, since the incident angle to the object surface varies corresponding to the wavelength, when field curvature of the f.theta. lens or mechanical error causes a partial out-focus area, a plurality of beam spots are formed at the different points in the out-focus area.
Since the spot separation seriously deteriorate the drawing performance of the plotter, the conventional optical system includes a filter that selects one peak wavelength. The filter cuts the peak wavelengths other than the selected peak wavelength, which lowers the energy efficiency.
Japanese Patent Provisional Publication No. Sho 61-193130 discloses the optical system including a wedge prism located at the front and/or rear of the AOM. Since the chromatic dispersion of the AOM can be canceled by the chromatic dispersion of the wedge prism, the optical paths of the laser beams of different wavelengths coincide with each other.
However, in the optical system of the publication, since the wedge prism is disposed in the divergent or convergent laser beam, astigmatism and coma occur.
Further, since the correction effect is fixed by the specification of the wedge prism, the chromatic dispersion of the wedge prism should be accurately defined to coincide the optical paths, which narrows tolerance to the specification of the wedge prism, increasing manufacturing time.